Frequency shift keying has been commonly utilized to transmit data at voice band frequencies over telephone lines. The voice band information in turn, may be transmitted as a PCM signal. Thus, a data signal will be initially modulated to an FSK signal and later further encoded into PCM.
While the data signal could be decoded in two stages (from PCM to FSK to data) it would be desirable to directly convert from PCM to data. Discrimination between the marks and spaces of a received PCM encoded FSK data signal might be done by digitally counting the number of digital samples between successive zero-crossings. However, to do so the minimum difference in the periods between the two FSK signals must be at least twice the sampling period of the PCM signal. A standard voiceband PCM signal is sampled at an 8 KHz rate and thus has a period of 0.125 msec. One FSK standard utilizes frequencies having a period of 0.934 msec. (1070 Hz) and 0.787 msec. (1270 Hz) so that the difference in their periods is 0.147 msec. To discriminate between these two frequencies the maximum sampling period of the PCM signal must be less than one-half this difference (i.e. 0.147/2=0.073 msec.) much less than the PCM standard of 0.125 msec. It will be evident therefore that a more precise determination of the zero-crossings must be made in order to directly determine the data from the PCM signal.